The present invention relates to hydraulic lifting equipment such as a portable floor jack of the type having a swinging lift arm, used in automobile repair shops; and forklift trucks of the manually operated type having a low-rise or a high-rise lifting capacity.
One object of the invention is to provide a piston-and-cylinder arrangement for the foregoing types of lifting apparatus in which a passage is provided at the inner end of the piston which communicates with a bypass opening in the bore supporting the piston only when the piston has reached its limit of travel. The bypass opening communicates with the hydraulic fluid reservoir so that when the piston has reached its limit of movement, further operation of the fluid has no effect on the piston, but merely cycles the fluid through the passage in the piston, the bypass opening and back into the reservoir. Annular fluid seals are installed in the bore on both sides of the bypass opening so that fluid does not enter the opening when the piston is at any intermediate point in its travel and cannot leak out of the bore when fluid is being bypassed at the outer end of piston travel.
Such an arrangement eliminates the necessity for providing a complex movable valve mechanism on the inner end of the piston, such as is shown in U.S. Pat. No. 3,404,868, or a spring operated bypass valve mechanism such as is shown in U.S. Pat. No. 3,907,252. The mechanism in these prior patents require the use of movable elements and additional springs which can malfunction after repeated use and, in any event, add to the cost of fabrication.
Another object of the invention is to provide a piston-and-cylinder actuator for lifting devices in which only a short length of the cylinder is machined to closely fit the piston. The length of the piston itself may greatly exceed the length of the bore itself, the remainder of the cylinder comprising a chamber which surrounds the piston when retracted but in outwardly spaced relationship to it, whereby the inner surface of that enlarged portion of the cylinder need not be carefully machined to the same tolerances as would be otherwise required.
A further object which is achieved by the aforementioned design is that by eliminating the necessity for a radial flange at the inner end of the piston it can be inserted into the bore of the cylinder from the outside and by having an internal passage which cooperates with a bypass opening in the bore, there is no danger that the piston will be later displaced from the cylinder, as a result of continued pumping of hydraulic fluid and regardless of the fact that there is no physical obstacle preventing its removal.
Another object of the invention is to provide a closure member for one end of a double walled tubular chamber, the inner tubular element comprising a cylinder for an actuator piston and the outer tubular element defining with the first element a reservoir for hydraulic fluid for actuating the piston. The closure member is provided with a central bore to receive the actuator piston and the bore includes a radial bypass opening for communication with the space between the tubular elements. The piston is provided with a bypass passage to communicate with the opening when projected to the full length and, because it may be of uniform diameter along its entire length, the same closure member may be combined with tubular elements of any length corresponding with the length of piston necessary to provide the desired length of travel for the apparatus in question.
The application discloses, but is not limited to, a closure member of the type described for cylinder and piston actuating a portable floor jack wherein the piston travel is relatively short; and two types of manually operated forklifts in which the piston travel is of varyingly greater extent in each case.
Other objects and advantages would be apparent to those skilled in the art after reading the following description in connection with the annexed drawings.